@rustonm Welcome to the forum.
I am confused as to what you want to do. The landline 'spending cap' is the amount that you can spend in the month on calls. If you have a call plan then it is for any 'out of plan' calls. If you are paying for calls 'as you go' then it is all calls.
If this limit could be set to £0 (I don't know the answer to this, you will probably need to wait until a Plusnet staffer comes along), this means that no landline calls can be made. The easiest way to control this is to not plug in a phone.
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Maybe the line is only wanted for incoming calls.
Hi
See https://community.bt.com/t5/Archive-Staging/Block-all-outgoing-calls/td-p/1545180
PN may have the same, but it is not free
Dave
There’s a home phone credit threshold https://www.plus.net/help/phone/about-phone-credit/ which we might be able to lower to £0, although it won’t prevent the first call as it can take some time to kick in.
Also when the threshold is reached, you’d be charged £5.76 to bar the line, which is automatically lifted when we receive the payment for the call.
So you’d be better off adding “Call Barring” as a feature on your line, which is free of charge, rather than trying to set a threshold. You can do this online through your account and once activated, you can then configure it to bar outgoing calls. We explain how to in more detail here https://www.plus.net/help/phone/call-features/